1. Field of the Invention
This invention relates to a luminescent material, a method of manufacturing the luminescent material, and a light-emitting device.
2. Description of the Related Art
A light-emitting diode (LED) is constituted by a combination of an LED chip a light source for excitation and a luminescent material and is enabled to emit various colors depending on this combination. In the case of a white LED which is enabled to emit white light, there is employed a combination of an LED chip with a luminescent material. For example, there is employed a combination of an LED chip emitting mainly a light of blue region with a yellow luminescent material. There is also employed a combination of an LED chip emitting a light of ultraviolet or near-ultraviolet region with a mixture of luminescent materials. This mixture of luminescent materials is composed of a blue luminescent material which emits luminescence of blue color, a green-yellow luminescent material which emits luminescence of green-yellow color, and a red luminescent material which emits luminescence of red color. The luminescent material to be used in a white LED is required to be not only capable of effectively absorbing the light ranging from near-ultraviolet region to blue region (310-420 nm) which corresponds to the emission wavelength of LED chip acting as an excitation light source but also capable of efficiently emitting visible light.
There has been proposed a white LED wherein a bivalent europium-activated alkaline earth orthosilicate luminescent material is employed. This luminescent material has a composition represented by (Sr, Ba, Ca)2SiO4:Eu and is capable of emitting a light having an optional peak wavelength that can be realized by changing the content of the alkaline earth metal element. However, since the white LED of this kind is employed in combination with a luminescent material having a broad-band emission spectrum with a half band width of 80 nm or more, the white LED is limited in color rendering.
Further, there has been also proposed a Dy-activated luminescent material. However, since the luminescent material of this kind can be hardly excited by the light ranging from an ultraviolet region to a blue region and since Dy3+ of the Dy-activated luminescent material is generally incapable of exhibiting an excitation band in a region ranging from an ultraviolet region to a blue region, the luminescent material is designed to be excited not by the light ranging from an ultraviolet region to a blue region but by vacuum ultraviolet rays or electron beam.